Use of multi-layer piezoelectric element made by alternately stacking piezoelectric layers and internal electrodes, for example, has been proposed for constituting a multi-layer electronic component in order to achieve a large displacement by making use of electrostrictive effect. The multi-layer piezoelectric element can be divided into two categories: fired-at-once type and stacked type where piezoelectric porcelain and internal electrode sheet are stacked one on another alternately. When the requirement to reduce the voltage and manufacturing cost are taken into consideration, the multi-layer piezoelectric element of fired-at-once type is more advantageous in order to decrease the layer thickness.
The multi-layer piezoelectric element of fired-at-once type is made similarly to a multi-layer ceramic capacitor by forming an active section by stacking green sheets that include a piezoelectric material and internal electrode sheet that includes an internal electrode material, forming an inactive section by stacking a plurality of the ceramic green sheets on the top and bottom surfaces of the active section, and degreasing and firing the stack.
In recent years, such a practice has been becoming common that a compact multi-layer piezoelectric actuator is operated continuously over an extended period of time with an electric field of higher intensity being applied thereto, in order to obtain a larger displacement under a high pressure.
In order to meet such a requirement, a multi-layer electronic component disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-299588 has column-like portion formed in an internal electrode layer from a material, that includes 10 to 20% of piezoelectric ceramic powder of which particle size is controlled within a range form ½ to 1 times the thickness of the internal electrode, so as to bridge between ceramic layers, thereby to prevent delamination of the internal electrode and the ceramic layer after firing.
However, in the multi-layer electronic component disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-299588, because of slow rate of cooling down after heat treatment in the process of connecting the internal electrode and an external electrode, there is a gap generated between the internal electrode and the ceramic layer over substantially the entire interface thereof in an area where column-like portion 51 is not formed, due to a difference in thermal expansion between the internal electrode 102 and the ceramic layer 101 as shown in FIG. 5, with the gap T larger than 2 μm in 50% or more of the entire interface. As a result, there has been such a problem that continuous operation over a long period in high electric field leads to delamination.
In a multi-layer electronic component disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-217796, occurrence of micro cracks that are generated during cutting and may cause shorting is prevented by subjecting a cut surface (external electrode forming surface) formed by mechanical processing of the device to a heat treatment that is applied at a temperature higher than that of the first firing.
However, because of a slow rate of cooling down carried out after the heat treatment applied at a temperature higher than the firing temperature, there has been peel-off occurring over substantially the entire interface thereof due to a difference in thermal expansion between the internal electrode and the ceramic layer. As a result, there has been such a problem that continuous operation over a long period in high electric field leads to delamination.
FIG. 7 shows a multi-layer piezoelectric element disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-133715, which is constituted from a stack 200 and external electrodes 223 formed on a pair of side faces that oppose each other. While the stack 200 is formed by stacking piezoelectric material 221 and internal electrode 222 one on another, the internal electrode 222 is not formed over the entire principal surface of the piezoelectric material 221, in a so-called partial electrode structure. The piezoelectric material is stacked such that the internal electrode 222 is placed in every other layer in a staggered manner so as to be exposed alternately at the left then at the right on different side faces of the stack 200. Then the external electrodes 223 are formed so that the internal electrodes 222 that are exposed to the pair of opposing side faces of the stack 200 are connected to each other, thereby connecting the internal electrodes 222 in every other layer.
The multi-layer piezoelectric element of the prior art may be manufactured by printing an internal electrode paste in the pattern of a predetermined electrode structure on a ceramic green sheet that includes the material of the piezoelectric material 221, stacking a plurality of the green sheets coated with the internal electrode paste so as to form a multi-layer green compact and firing the green compact thereby to make the stack 200. Then the external electrodes 23 are formed by firing on a pair of side faces of the stack 200, thereby to make the multi-layer piezoelectric element.
The internal electrode 222 is formed from an alloy of silver and palladium and, in order to fire the piezoelectric material 221 and the internal electrode 222 at the same time, composition of metals included in the internal electrode 222 was set to 70% by weight of silver and 30% by weight of palladium (refer to, for example, Japanese Unexamined Utility Model Publication (Kokai) No. 1-130568).
The internal electrode 222 is made of metal composition that includes silver-palladium alloy instead of pure silver because, when a voltage is applied between the pair of opposing internal electrodes 222 that are made of silver without palladium content, the so-called silver migration occurs in which silver atoms of the pair of internal electrodes 222 propagate along the device surface from the positive electrode to the negative electrode. Silver migration occurs conspicuously in an atmosphere of high temperature and high humidity.
In case a multi-layer piezoelectric element of the prior art is used as a piezoelectric actuator, it may be provided with lead wires (not shown) soldered onto the external electrodes 223 and operated by applying a predetermined voltage between the external electrodes 223. In recent years, since it is required to make a compact multi-layer piezoelectric element capable of achieving a large amount of displacement under a high pressure, continuous operation is carried out over a long period with a higher electric field applied.
Such a multi-layer piezoelectric element of fired-at-once type as described above is required to equalize the temperature at which the internal electrode 222 is sintered and the temperature at which the piezoelectric material 221 is sintered, and compositions of the materials used to form the internal electrode 222 and the piezoelectric material 221 have been studied. However, since this allows residual stress caused by the difference in thermal expansion between the internal electrode and the ceramic layer to be concentration in the crystal grains of the piezoelectric material 221 that faces the internal electrode 222, there has been such a problem that delamination occurs in which the internal electrode 222 peels off the piezoelectric material 221 during operation when the device is used as an actuator.
Particularly when those among the crystal grains of the piezoelectric material 221 that face the internal electrode 222 are small, such troubles occur as the value of dielectric constant becomes smaller than that of larger grains of the same composition due to the size effect, and the amount of piezoelectric displacement becomes smaller. Even when average crystal grain size of the crystal grains of the piezoelectric material 221 is made larger, if there are grains having small amount of piezoelectric displacement among the crystal grains of the piezoelectric material 221 that faces the internal electrode 222, smaller amount of displacement than that of the crystal grains of the piezoelectric material 221 during operation causes the residual stress generated due to the difference in thermal expansion between the internal electrode 222 and the ceramic layer 221 to be concentrated at one point thus making an initiating point for cracks and delamination.
There has also been such a problem that the mount of displacement varies due to the occurrence of delamination. When the rate of occurrence of delamination becomes higher, temperature of the device increases. When the heat generated by the device exceeds the heat that can be removed by dissipation, thermal excursion occurs, resulting in breakage and sudden failure to achieve the required amount of displacement. Therefore, there has been a demand for internal electrodes having lower specific resistance in order to suppress the device temperature from rising.
However, specific resistance of the silver-palladium alloy that has been used in the prior art is higher than that of a single-element metal such as silver or palladium depending on the composition of the alloy. For example, silver-palladium alloy including 70% by weight of silver and 30% by weight of palladium has specific resistance 1.5 times as high as that of palladium. Moreover, lower sintering density of the internal electrode 222 results in even higher resistance, thus posing a limitation to decreasing the specific resistance of the internal electrode 222.
When the conventional multi-layer piezoelectric element is used as an actuator for driving a fuel injection apparatus or the like as described above, there is a problem that the amount of displacement gradually changes resulting in malfunction of the apparatus. Therefore, it has been called for to suppress the amount of displacement from changing and improve durability during continuous operation over a long period.
For a multi-layer piezoelectric element, it is a common practice to carry out polarization treatment by applying a voltage of about 1 kV (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-293625). Specifically, in the polarization treatment disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-293625, the multi-layer piezoelectric element having the external electrodes formed thereon is (1) immersed in a heated oil bath, (2) subjected to a voltage applied thereto, and (3) cooled down after decreasing the voltage.
However, in the polarization treatment disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-293625, there is a problem that the crystal grains that constitute the piezoelectric material cannot undergo fully saturated polarization and, for example, particularly the amount of displacement among the piezoelectric characteristics decreases in an operation test conducted over a long period of time. This is because the degree of orientation of the crystal grains of the piezoelectric material changes more significantly through operation.